System and method for limiting cyclic control inputs

ABSTRACT

A control system having a first loop configured to provide a longitudinal blowback value of a rotor blade during flight and a second loop associated with the first loop, the second loop being configured to provide a design maximum total flapping value and a lateral flapping value. A method includes calculating a flight control limit from the design maximum total flapping value and the lateral flapping value. An upper longitudinal cyclic control limit is calculated by adding the flight control limit to the longitudinal blowback value. A lower longitudinal cyclic control limit is calculated by subtracting the flight control limit from the longitudinal blowback value.

BACKGROUND

1. Field of the Present Description

The present application relates generally to flight control systems, andmore specifically, to an aircraft flight control system for rotor bladeflapping.

2. Description of Related Art

All rotor systems are subject to dissymmetry of lift in forward flight.During hover, the lift is equal across the entire rotor disk. As thehelicopter gains airspeed, the advancing rotor blade develops greaterlift because of the increased airspeed. For example, rotor blades athover move at 300 knots and in forward flight at 100 knots the advancingblades move at a relative speed of 400 knots and while the retreatingblades move at 200 knots. This has to be compensated for in some way, orthe helicopter would corkscrew through the air doing faster and fastersnap rolls as airspeed increased.

Dissymmetry of lift is compensated for by blade flapping. Because of theincreased airspeed (and corresponding lift increase) on the advancingrotor blade, the rotor blade flaps upward. Decreasing speed and lift onthe retreating rotor blade causes the blade to flap downward. Thisinduced flow through the rotor system changes the angle of attack on therotor blades and causes the upward-flapping advancing rotor blade toproduce less lift, and the downward-flapping retreating rotor blade toproduce a corresponding lift increase. Some rotor system designs requirethat flapping be limited by flapping stops which prevent damage to rotorsystem components by excessive flapping. In addition to structuraldamage, aircraft control can be compromised if the rotor flaps into thestop. Thus it becomes incumbent on the aircraft designer to controlflapping and warn of this hazardous condition. This applicationaddresses this requirement.

Conventional devices and methods to control flapping include providing adisplay showing the longitudinal stick position of the aircraft. In oneembodiment, the display is a simple green tape that grows from a centerposition. Tic marks associated with the display represent 10 percentcontrol margin remaining. Common problems associated with thisconventional device include: there is no interface to display thecontrol power remaining before a hazardous flapping condition isreached. Although the foregoing developments represent great strides inthe area of aircraft displays, many shortcomings remain.

DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the application are setforth in the appended claims. However, the invention itself, as well asa preferred mode of use, and further objectives and advantages thereof,will best be understood with reference to the following detaileddescription when read in conjunction with the accompanying drawings,wherein:

FIG. 1 is a side view of a rotary aircraft;

FIG. 2 is an oblique view of a tiltrotor aircraft;

FIGS. 3A and 3B are oblique views of a rotary system;

FIGS. 4A-4C are front views of a display of the control system accordingto the preferred embodiment of the present application;

FIG. 5 an enlarged view of a portion of the display of FIG. 4A taken atVI-VI;

FIG. 6 is a schematic of the flight control system according to thepreferred embodiment of the present application;

FIG. 7 is a flow chart depicting the preferred method according to thepreferred embodiment of the present application;

FIG. 8 is a schematic of the control power management subsystem (CPMS);and

FIG. 9 is a flow chart depicting the preferred method.

While the system and method of the present application is susceptible tovarious modifications and alternative forms, specific embodimentsthereof have been shown by way of example in the drawings and are hereindescribed in detail. It should be understood, however, that thedescription herein of specific embodiments is not intended to limit theinvention to the particular embodiment disclosed, but on the contrary,the intention is to cover all modifications, equivalents, andalternatives falling within the spirit and scope of the process of thepresent application as defined by the appended claims.

DETAILED DESCRIPTION

The system and method of the present application overcomes theabovementioned problems commonly associated with conventional aircraftcontrol systems. The control system comprises a subsystem adapted tomodifying predetermined flight control limits for a particular aircraft.The subsystem determines whether the aircraft is operating within ornear an impending hazardous flight condition, which, in the exemplaryembodiments, are conditions where excessive blade flapping occurs. Thecontrol system further comprises a display having a symbol, i.e., apipper, which identifies displacement of the pilot's cyclic controllercombined with pitch control feedbacks and/or pedal displacement and yawcontrol feedbacks relative to the flight control limits. Furtherdescription and illustration of the control system and method isprovided in the figures and disclosure below.

It will of course be appreciated that in the development of any actualembodiment, numerous implementation-specific decisions will be made toachieve the developer's specific goals, such as compliance withsystem-related and business-related constraints, which will vary fromone implementation to another. Moreover, it will be appreciated thatsuch a development effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking for those of ordinary skill in theart having the benefit of this disclosure.

Referring now to the drawings, FIGS. 1 and 2 show two different rotaryaircraft utilizing the flight control system of the present application.FIG. 1 shows a side view of a helicopter 101, while FIG. 2 shows anoblique view of a tiltrotor aircraft 201. The flight control system ispreferably utilized in tiltrotor aircraft 201 during low speeds and witha fixed lateral cyclic. However, it will be appreciated that the controlsystem is easily and readily adaptable for use with other types ofrotary aircraft, i.e., helicopter 101, operating at various speeds andwith or without a fixed lateral cyclic control.

Helicopter 101 comprises a rotary system 103 carried by a fuselage 105.One or more rotor blades 107 operably associated with rotary system 103provide flight for helicopter 101 and are controlled with a plurality ofcontrollers within fuselage 105. For example, during flight a pilot canmanipulate the cyclic controller 109 for changing the pitch angle ofrotor blades 107, thus providing lateral and longitudinal flightdirection, and/or manipulate pedals 111 for controlling yaw direction.The system of the present application is preferably carried withinfuselage 105, thereby providing viewing access to the pilot duringflight.

Tiltrotor aircraft 201 includes two or more rotary systems 203 carriedby rotatable nacelles. The rotatable nacelles enable aircraft 201 totakeoff and land like a conventional helicopter, thus the rotary systemsof tiltrotor 201 are susceptible to excessive flapping of the rotorblades 205 caused by control of the rotor blades, rotor system rotation,and the rotor operating environment such as wind speed and direction. Inthe preferred embodiment, the control system of the present applicationis carried within fuselage 207 for assisting the pilot during flight. Itshould be understood that, like helicopter 101, tiltrotor aircraft 201comprises a cyclic controller and pedals for manipulating lateral,longitudinal, and yaw control.

For ease of description, some of the required systems and devicesoperably associated with the present control system are not shown, i.e.,sensors, connectors, power sources, mounting supports, circuitry,software, and so forth, in order to clearly depict the novel features ofthe system. However, it should be understood that the system of thepresent application is operably associated with these and other requiredsystems and devices for operation, as conventionally known in the art,although not shown in the drawings.

Referring to FIGS. 3A and 3B in the drawings, oblique views of rotarysystem 103 are shown. FIG. 3A shows rotary system 103 during normaloperation, while FIG. 3B shows rotary system 103 during hazardous flightconditions, i.e., the rotary system experiencing excessive flapping.Rotary system 103 comprises a mast 301 rotatably attached to rotorblades 107 via a rotor yoke 303. One or more restraints 305 and/or othernearby structures are positioned alongside mast 301. In the exemplaryembodiment, restraints 305 are conventional “stops” adapted to restrainthe movement of the hub. It should be understood that both helicopter101 and tiltrotor 201, along with other types of rotary aircraft, aresusceptible to excessive flapping, which could result in damage to therotary system.

During flight, the rotation of mast 301 combined with the pitching ofrotor blades 107 causes flapping, as depicted with vertical arrows.Excessive flapping can cause yoke 303 to tilt in direction D1, asindicated with the vertical arrow, which in turn could cause the yoke tocome into contact with restraint 305, resulting in damage to componentsof the rotor system and/or restraint 305, and in some scenarios,resulting in catastrophic failure. It will be appreciated that one ofthe novel features of the control system of the present application isto assist the pilot in controlling flight of the aircraft to avoidcontact between yoke 303 and restraint 305.

Referring now to FIGS. 4A-4C in the drawings, control system 401according to the preferred embodiment of the present application isshown. System 401 comprises a display 403 for displaying flight controllimits on a screen. FIG. 4A shows system 401 during normal flight whencertain portions of the design flight control envelope are limited by acontrol power management subsystem (CPMS), while FIG. 4C shows theflight control envelope being morphed as the aircraft approacheshazardous flight conditions. FIG. 4B shows the transition, i.e.,morphing of the flight control envelope, as the aircraft moves relativeto hazardous flight conditions.

Display 403 is provided with a symbol 405, i.e., a pipper, which, in thepreferred embodiment, displays displacement of the cyclic controller 109and pedal 111. In the preferred embodiment, vertical pipper motion ondisplay 403 represents the symmetric cyclic or, equivalently, thedisplacement of the longitudinal cyclic controller 109, while horizontalpipper motion on the display 403 represents the differential left-rightrotor cyclic, or equivalently, control pedal 111. However, it will beappreciated that alternative embodiments of display 403 could easily beadapted to include other flight parameters and/or different controllermovement in lieu of the preferred embodiment. For example, system 401could be adapted to display a symbol indicating movement of both thecyclic lateral and the cyclic longitudinal movement in lieu of thepreferred embodiment. Symbol 405 cues the pilot as to the cyclic stickor pedal inputs required to increase the margin from the impendinghazardous condition. It should be appreciated that the pipper positionin FIG. 4B cues the pilot that left pedal and aft stick will increasethe control margin.

It should be understood that display 403 is adapted to display both yawand pitch control of the aircraft. For example, the vertical axis ofdisplay 403 represents the pitch control relative to manipulation of thecyclic controller 109, while the horizontal axis of display 403represents the aircraft yaw control relative to manipulation of pedal111. Display 403 provides significant advantages by displaying both yawand pitch control relative to the control limits.

FIG. 4A shows display 403 having a flight control envelope 407 definedby the aircraft control limits 409, represented as a solid line. Itshould be understood that control limits 409 are either design flightlimits established for the particular flight capabilities of theaircraft or limits imposed by the CPMS. For example, other rotaryaircraft could include flight control limits having a smaller generallyrectangular shape profile in lieu of the larger octagonal shape profileof the preferred embodiment. It should be appreciated that display 403is adapted to display any flight control limit of the rotary aircraft.

Flight control envelope 407 comprises a first region 411, wherein theflight control limits are not modified by CPMS, as will be explainedmore fully below. Flight control envelope 407 further comprises a secondregion 413, specifically, a total of four of second regions 413 aredisposed within region 411. In the exemplary embodiment, region 413 isdefined with a dashed line 415. In region 413, the aircraft is operatingin or near impending hazardous conditions, i.e., excessive flapping, andthe flight control limits are modified by CPMS.

FIG. 4B shows first flight control envelope 407 transitioning to asecond flight control envelope 417. The morphing of first controlenvelope 407 occurs when the aircraft nears impending hazardous flightconditions. In the second flight control envelope 417, region 411remains unaffected by CPMS. It should be understood that display 403continuously and interchangeably displays transition between envelopes407 and 417.

FIG. 4C shows a third flight control envelope 419, which is an ultimateshape of display 403 during impending hazardous flight conditions,wherein the entire available control envelope is limited by CPMS. Flightcontrol envelope 419 includes a dashed line 421 forming a region thereinfor cueing the pilot to exercise caution to avoid flight control limits.The region delineates a safe margin for controlling the aircraft withoutconsideration of approaching an unsafe operating condition.

It should be understood that the flight control envelopes disclosedherein are generated by the aircraft control limits modified by controllimits established by CPMS, which are continuously calculated based uponblade flapping and actuator movement. Thus, the general shape and sizeof the envelopes vary. For example, in FIG. 5, region 413 is shownhaving a width W, which increases in length during high blade flappingand decreases in length with low blade flapping. Such features enablethe pilot to effectively manipulate the controllers to avoid excessiveflapping.

It should also be appreciated that Display 403 continuously transitionsbetween flight control envelopes 407 and 419 depending on theconstraints imposed by CPMS, wherein flight control envelope 407represents minimal CPMS limiting while envelope 419 represents maximalCPMS limiting. It should be understood that FIG. 4B is one of manypossible flight envelopes created as the aircraft transitions betweennormal flight, i.e. first flight control envelope 407, to an impendinghazardous condition, i.e., third fight control envelope 419. It shouldbe noted that the horizontal and vertical lines of flight control limits409 changes during transitioning between envelopes. For example, acomparison of FIGS. 4A and 4B illustrates flight control limits 409having a shorter horizontal and vertical length as the flight envelopemorphs when the aircraft approaches impending hazardous flightconditions.

Both flight control envelopes 407 and 419 create respective octagonaland diamond geometric shapes in the exemplary embodiments. Of course, itshould be appreciated that alternative embodiments could includedifferent geometric shapes depending on the desired limits and dependingon the flight characteristics of the aircraft.

Referring to FIG. 6 in the drawing, a schematic view of flight controlsystem 401 is shown. System 401 further comprises a flight controlsubsystem (FCS) 601 and a control power management subsystem 603 (CPMS).Both FCS 601 and CPMS 603 are operably associated with one another toassist the pilot to avoid excessive flapping.

Box 605, labeled as flight control laws (CLAW), depicts the outcomeflight control limits generated by both FCS 601 and CPMS 603. As isshown, a solid line represents the original flight control limits, whilethe dashed line represents the modified flight control limits, i.e., thesolid line being lowered with application of CPMS 603. It should beunderstood that CPMS 603 only limits the flight control limits while theaircraft is flying in or near impending hazardous flight conditions,i.e., excessive blade flapping. The modified flight control limits arethereafter displayed with display 403.

In the preferred embodiment, pilot controller commands 607, i.e., fromcyclic controller 109 and/or pedal 111, along with automatic aircraftcontrols 609, are received by FCS 601, then relayed to aircraftactuators 611. The positioning of the actuators 611 are shown by symbol405 on display 403.

CPMS 603 is preferably operably associated with a first sensor 613adapted to sense displacement movement of actuators 611 and a secondsensor 615 adapted to sense blade flapping of rotary system 103. CPMS603 is provided with a flapping limiting algorithm, which receivessensed data from both sensor 613 and sensor 615 to generate controllimit envelopes (See, FIGS. 4A-4C). As discussed, the flapping magnitudeand actuator displacement changes during flight, thus resulting inchanging control limits generated by CPMS 603.

Referring to FIG. 7 in the drawings, a flowchart 701 depicting thepreferred method is shown. Box 703 shows the first step, which includesgenerating control limits for the aircraft, which are predeterminedcontrol limits for the particular aircraft. In the preferred method, thecombined commanded pilot controls and the automatic aircraft controlsare limited by the flight control margins. Box 705 depicts the nextstep, which includes modifying the control limits to avoid impendinghazardous conditions, i.e., excessive flapping. This step is achievedwith CPMS via a flapping limiting algorithm operably associated with theaircraft rotary system and the aircraft actuators. A display is providedto display the flight control envelope defined with the flight controllimits, as depicted in box 707. A symbol is also utilized to show thecontroller displacement relative to the control limits. The next stepmorphing the envelope as the aircraft approaches impending hazardousflight conditions, as depicted in box 709.

Turning next to FIG. 8 in the drawings, a schematic view of CPMS 603 isshown. The CPMS comprises one or more of a sensor for determiningflapping and an algorithm configured to set control limits to preventrotor flapping into mechanical stops or exceeding a design flappinglimit by computing dynamic limits on longitudinal and lateral cycliccontrol inputs. In the preferred embodiment, the control system isutilized on tiltrotor aircraft which has a fixed lateral. However, itwill be appreciated that the control system is configured for use withdifferent types of rotary aircraft with variable control of both lateraland longitudinal cyclic.

FIG. 8 (subsystem 800) provides a detailed view of the algorithmutilized with subsystem 603. In particular, the algorithm is implementedin the flight control system software and receives data such asairspeed, longitudinal and lateral flapping, and the position of thelateral and longitudinal cyclic actuators as inputs. Thereafter, thealgorithm generates CPMS-based cyclic control limits which may in turnlimit the cyclic control commands of the flight control system. Itshould be appreciated that the algorithm is repeated for each rotor whenimplemented on a tiltrotor aircraft.

Subsystem 800, which is preferably subsystem 603, comprises one or moreof a first control loop 801 configured to determine a longitudinalblowback value, a second control loop 803 configured to determine adesign maximum total flapping value and a lateral flapping value, and athird control loop 805 configured to determine a lateral blowback valueand value of design maximum magnitude of lateral flapping. In thepreferred embodiment, control loop 805 is an optional control loop. Forexample, in some embodiments the rotary aircraft could include fixedlateral cyclic controlling. However, it should be appreciated thatcontrol system 401 is configured to include an algorithm for bothlateral and longitudinal cyclic limiting, as depicted in FIG. 8.

Control loop 801 receives sensed longitudinal cyclic data from theactuator sensor 613 and longitudinal flapping data from the flappingsensor 615. The sensed data are summed to generate longitudinal blowbackand then passed through a low pass noise filter 807, to create a laggedblowback value. Equation (1) shows the longitudinal blowback value.BB _(long) =a ₁ +B _(1C)  (1)

where BB_long is the longitudinal component of blowback, a_(—)1 is thelongitudinal component of flapping, and B_(—)1C is the longitudinalcomponent of the cyclic control.

It should be understood that blowback is a phenomenon affecting therotor of a helicopter as it overcomes dissymmetry of lift throughflapping. In forward flight, rotor blades experience more lift as theyrotate forward. This increased lift is a result of an increased relativespeed causing the blade to flap up and decrease its angle of attack. Asthe blade continues to rotate, it achieves its maximum upward flappingdisplacement over the nose of the aircraft and maximum downward flappingdisplacement over the tail. This results in the rotor disk being tiltedto the rear and is referred to as blowback, as if the rotor disk hadflapped or tilted back, or as if it had been blown back by the relativewind. The effect is more pronounced at higher airspeeds but more easilyrecognized as the aircraft accelerates to translational lift airspeedsfrom a hover. Blowback results in a slowing of the aircraft and thepilot counters the effect by applying forward input to the cycliccontrol.

Control loop 803 is configured to determine a design maximum totalflapping Fmax value and a lateral component of flapping value.Specifically, control loop 803 receives input variables from a thirdsensor 809, e.g., airspeed or orientation of the tiltrotor nacelles,which in turn are compared to flight test input parameters in table 811.The compared input variables determine a design maximum total flappingFmax value. The square value of Fmax is a upper flight control limit, asshown in diagram 814. Diagram 814 includes a lower flight control limitcreated by bias 815, which is preferably a zero value. Equations (2) and(3) show the values for the upper and lower limits, respectively, fordiagram 813.UL=F _(max) ²  (2)LL=0  (3)

where UL is the upper limit, LL is the lower limit, and F_max is thedesign maximum total flapping.

In the exemplary embodiment, table 811 includes a plurality ofdesignated values for determining total flapping. During aircraftdevelopment, table 811 is preferably operably associated with a devicepositioned in the cockpit of the aircraft, which can be manuallyadjusted during flight. It will be appreciated that alternativeembodiments could include Fmax tables that are autonomously adjusted bya flight control system. Further, the ultimate alternative embodiment isfixed Fmax values determined by the preferred aircraft developmentembodiment.

It should be noted that F_max is a function of aircraft variables (e.g.,airspeed, nacelle) and tuned using empirical data and knowledge of theaccuracy of the flapping measurements and the flapping stop limit. Inthe preferred aircraft development embodiment, F_max is the only tuningparameter required to guarantee that the design maximum total flappingvalues in Table 811 will be constant for all combinations of BB_long and

Ideally, the single tuning parameter of the algorithm, Fmax, would beset to the design flapping limit. In practice, however, Fmax must be setto be less than the design limit based on considerations of flappingmeasurement accuracy and flight test results. In the preferredembodiment, Fmax is generally a function of airspeed. However, it willbe appreciated that Fmax could be a function of other flight parameters.With provisions in the developmental flight control system to varyparameters in flight, Fmax can be rapidly and efficiently tuned toaccommodate the flapping occurring in the worst case maneuvers expectedof the aircraft.

Referring back to FIG. 8, the lateral component of flapping is sensedfrom flapping sensor 615, which later passes through a low pass filter814. Thereafter, the lateral component of flapping is squared andsubtracted from the Fmax squared to calculate an input control value. Itshould be noted that the input control value for diagram 813 cannotexceed the upper limit (UL in equation (2)) established by Fmax squaredand cannot be lower than the lower limit (LL in equation (3))established by bias 815. The output value from diagram 813 is squaredprior to creating an upper CPMS-based longitudinal cyclic limit 817 anda lower CPMS-based longitudinal cyclic limit 819. Equations (4) and (5)show the input limit value and the output limit value, respectively.IN=F_(max) ² −b ₁ ²  (4)OUT=lim(F _(max) ² −b ₁ ²)  (5)

where b_(—)1 is the lateral component of flapping, IN is the inputcontrol limit value, and OUT is the output control limit value.

The upper CPMS-based longitudinal cyclic limit 817 is calculated as thesummation of BB_long and the square root value of OUT, while the lowerCPMS-based longitudinal cyclic limit 819 is calculated as BB_long lessthe square root value of OUT. Equations (6) and (7) show the upper andlower limits of CPMS-based longitudinal cyclic limits, respectively.

$\begin{matrix}{B_{1\;{UL}} = {{{BB}_{long} + \sqrt{OUT}} = {{BB}_{long} + \sqrt{\lim\left( {F_{\max}^{2} - b_{1}^{2}} \right)}}}} & (6) \\{B_{1\;{LL}} = {{{BB}_{long} - \sqrt{OUT}} = {{BB}_{long} - \sqrt{\lim\left( {F_{\max}^{2} - b_{1}^{2}} \right)}}}} & (7)\end{matrix}$

where B_(—)1UL is the upper CPMS-based longitudinal cyclic limit andB_(—)1LL is the lower CPMS-based longitudinal cyclic limit and wherea_(—)1, b_(—)1, and B_(—)1C are assumed to be available as sensedinputs.

The above equations can be rearranged to arrive at the followingequations (8)-(10). The first step requires the combination of F_max andCM_long equations and then to solve for B_(—)1LIM.F _(max) ²=(a ₁ +CM _(long))² +b ₁ ²  (8)

$\begin{matrix}{\mspace{79mu}{{CM}_{long} = {{{- a_{1}} \pm \sqrt{\left( {F_{MAX}^{2} - b_{1}^{2}} \right)}} = {B_{1\; C} - B_{1\;{LIM}}}}}} & (9) \\{B_{1{LIM}} = {{B_{1\; C} + {a_{1} \pm \sqrt{\left( {F_{MAX}^{2} - b_{1}^{2}} \right)}}} = {{BB}_{long} \pm \sqrt{\left( {F_{MAX}^{2} - b_{1}^{2}} \right)}}}} & (10)\end{matrix}$

where B_(—)1LIM=CPMS-based longitudinal cyclic command limit and CM_longis the control margin for longitudinal flapping.

It should be noted that the upper CPMS-based longitudinal cyclic limitis defined by the “+” sign on the SQRT function and the lower limit isdefined by the “−” sign.

Control loop 805 is an optional feature of system 401. Control loop 805includes a table 816, which receives one or more variables from sensor809 to determine a design maximum magnitude lateral component offlapping Fmaxlat. Control loop 805 utilizes the lateral component offlapping and lateral blowback to calculate the upper CPMS-based lateralcyclic limit 818 and the lower CPMS-based lateral cyclic limit 821.

The lateral blowback is the lateral component of flapping less thelateral cyclic. Equation (11) shows the lateral blowback.BB _(lat) =b ₁ −A _(1C)  (11)

where BB_lat is the lateral component of blowback and A_(—)1C is thelateral component of cyclic control.

The lateral component of blowback passes through a low-pass filter 823prior to being subtracted from the design maximum magnitude of lateralflapping Fmaxlat. Equations (12) and (13) show the upper CPMS-basedlateral cyclic limit 818 and the lower CPMS-based lateral cyclic limit821, respectively.A _(1UL) =F _(lat max) −BB _(lat) =F _(lat max)−(b ₁ −A _(1C))  (12)A _(1LL) =−F _(lat max) −BB _(lat) =−F _(lat max)−(b ₁ −A _(1C))  (13)

where A_(—)1UL is the upper CPMS-based lateral cyclic limit, A_(—)1 LLis the lower CPMS-based lateral cyclic limit, and F_latmax is the designmaximum magnitude of lateral flapping.

In the exemplary embodiment, the magnitude of F_latmax will be less thanthe magnitude of F_max and will be a function of aircraft flightvariables (e.g., airspeed, nacelle rotation) from sensor 809 and tunedusing empirical data and knowledge of the accuracy of the flappingmeasurements.

The above equations can be rearranged to arrive at equations (14)-(16)below.F _(lat max) =b ₁ +CM _(lat)  (14)CM _(lat) =F _(lat max) −b ₁ =A _(1LIM) −A _(1C)  (15)A _(1LIM) =A _(1C) −b ₁ +F _(lat max) =−BB _(lat) +F _(lat max)  (16)

where A_(—)1 LIM is the CPMS-based lateral cyclic command limit andCM_lat is the control margin for lateral flapping.

It should be noted that the design maximum magnitude of lateral flappingneeds to be considered for both positive and negative values of b_(—)1for upper and lower limit calculations, as shown and described above.

The control system is unique and novel in that it provides a simple,easily optimized, and effective method to control total flapping (havingboth longitudinal and lateral components) of a rotor with longitudinaland lateral cyclic control. The control system provides the requisitelimiting without compromising vehicle control. It can be modified toaccommodate independent lateral cyclic control. But, if it is decidedthat the additional control degree of freedom is not required, thenreductions in weight, cost, and complexity will be realized.

Referring next to FIG. 9 in the drawings, a flowchart 901 depicting thepreferred process of implementing the control system algorithm is shown.Boxes 903 through 911 depict the first steps of the process, whichincludes determining input values such as blowback and flapping. Thesevalues, including the process of obtaining them, are described in detailabove, and for simplicity, are not disclosed here. Thereafter, thecyclic control limits are calculated by utilizing the sensed inputblowback and flapping values, as depicted in boxes 913 and 915. Itshould be noted that boxes 909, 911, and 915 describe an optionalfeature of the preferred embodiment, wherein the upper and lower lateralcyclic limits are calculated when the lateral cyclic controlling is notfixed.

It should be noted that flowchart 901 depicts a broad overview of thepreferred process, and a detail overview of the preferred method isdiscovered when viewing FIG. 8 in conjunction with FIG. 9. For example,the process of determining the longitudinal blowback value, as depictedin box 903 of FIG. 9, is clearly shown in control loop 801 of FIG. 8.

It is apparent that a system and method having significant advantageshas been described and illustrated. The particular embodiments disclosedabove for a tiltrotor are illustrative only, as the embodiments may bemodified and practiced in different but equivalent manners apparent tothose skilled in the art having the benefit of the teachings herein. Itis therefore evident that the particular embodiments disclosed above maybe altered or modified, and all such variations are considered withinthe scope and spirit of the invention. Accordingly, the protectionsought herein is as set forth in the description. Although the presentembodiments are shown above, they are not limited to just theseembodiments, but are amenable to various changes and modificationswithout departing from the spirit thereof.

What is claimed is:
 1. A control system for a rotary aircraft,comprising: a first loop configured to provide a longitudinal blowbackvalue of a rotor blade during flight; and a second loop associated withthe first loop, the second loop being configured to provide a designmaximum total flapping value and a lateral flapping value; wherein thesquared value of the lateral flapping value is subtracted in the controlsystem from the squared value of the design maximum total flapping valueto create a fight control limit; wherein the flight control limit isadded to the longitudinal blowback value to create an upper longitudinalcyclic limit; and wherein the flight control limit is subtracted fromthe longitudinal blowback value to create a lower longitudinal cycliclimit.
 2. The control system of claim 1, further comprising: a displayconfigured to display a symbol identifying the displacement of thecontroller relative to the upper longitudinal cyclic limit and the lowerlongitudinal cyclic limit.
 3. The control system of claim 1, furthercomprising: a first sensor associated with the cyclic actuators of therotary aircraft, the sensor being configured to detect a displacement ofthe actuators; a second sensor associated with a rotor of the rotaryaircraft, the second sensor being configured to detect a lateralflapping movement and a longitudinal flapping movement of the rotorblade; and a third sensor associated with the rotary aircraft, the thirdsensor being configured to detect a flight parameter of the aircraft;wherein the first loop is associated with the first sensor and thesecond sensor; and wherein the second loop is associated with the secondsensor and the third sensor.
 4. The control system of claim 3, furthercomprising: a table of flapping values, the table being associated withthe second loop; wherein the flight parameter sensed by the third sensoris received by the table of flapping values and compared with thedesignated flapping values to determine the design maximum totalflapping value.
 5. The control system of claim 4, wherein the flightparameter is the aircraft airspeed.
 6. The control system of claim 4,wherein the table of design maximum flapping values is manually adjustedduring flight.
 7. The control system of claim 4, further comprising: athird loop operably associated with the first sensor and the thirdsensor, the third loop being configured to determine a lateral blowbackvalue created by the rotor blade during flight and a value of designmaximum magnitude of lateral flapping.
 8. The control system of claim 4,the third loop comprising: a table of lateral flapping values; whereinthe flight parameter sensed by the third sensor is received by the tableof lateral flapping values and compared with the designated flappingvalues to determine the design maximum magnitude of lateral flappingvalue.